Dual evaporator air conditioning systems are well known in the art for cooling air in front and rear portions of a cabin of a vehicle. A typical dual evaporator air conditioning system includes a primary HVAC unit to cool the front portion of the cabin and an auxiliary HVAC unit to cool the rear portion of the cabin. The primary HVAC unit includes a primary evaporator and the auxiliary HVAC unit includes an auxiliary evaporator. The primary and auxiliary evaporators are fluidly connected to a common compressor and common condenser. The compressor compresses and circulates refrigerant to the condenser. The condenser cools and condenses the refrigerant, which is then circulated to both the primary and auxiliary evaporators.
The primary evaporator is held by a primary housing and is used to transfer heat from the air to the refrigerant. A primary blower moves the air across the primary evaporator, and a plurality of primary air ducts direct the air into the front portion of the cabin. The auxiliary evaporator is held by an auxiliary housing and is used to transfer heat from the air to the refrigerant. An auxiliary blower moves the air across the evaporator, and a plurality of auxiliary air ducts direct the air into the rear portion of the cabin.
Examples of dual evaporator air conditioning systems are shown in U.S. Pat. No. 4,949,779 to Kenny et al. (the '779 patent) and U.S. Pat. No. 5,142,881 to Nagayama (the '881 patent). The dual evaporator air conditioning systems of the '779 and the '881 patents include primary and auxiliary evaporators connected to a common compressor to cool front and rear portions of a vehicle cabin.
Dual evaporator air conditioning systems of the prior art utilize a control system to control operation of the compressor and the primary and auxiliary HVAC units to cool the front and rear portions of the cabin. Generally, the control system activates the compressor when the primary HVAC unit is in a cooling mode, i.e., a user has requested cooled air for the front portion of the cabin. The auxiliary HVAC unit can also be in a cooling mode, i.e., the user has requested cooled air for the rear portion of the cabin. Alternatively, the auxiliary HVAC unit can remain in a non-cooling mode while the primary HVAC unit is in the cooling mode, i.e., the user has requested cooled air for the front portion, but not for the rear portion. In this instance, the compressor continues to circulate refrigerant through the auxiliary evaporator of the auxiliary HVAC unit even though the auxiliary HVAC unit is in the non-cooling mode. In such a case, liquid refrigerant and lubricating oil begin to accumulate in the auxiliary evaporator.
The liquid refrigerant and lubricating oil become stored or trapped in the auxiliary evaporator because the auxiliary evaporator is not transferring heat from the air in the rear portion of the cabin to the refrigerant in the auxiliary evaporator. As a result, the refrigerant is not converted to a vapor and the viscosity of the refrigerant in the auxiliary evaporator increases. As the viscosity of the refrigerant increases, more and more lubricating oil becomes trapped in the refrigerant to remain in the auxiliary evaporator. Accumulation of the liquid refrigerant and lubricating oil in the auxiliary evaporator results in refrigerant starvation to the rest of the system and poor compressor lubrication.
When liquid refrigerant is stored in the auxiliary evaporator, refrigerant for the rest of the dual evaporator air conditioning system is reduced. If the amount of liquid refrigerant that is stored is greater than a reserve charge, the primary evaporator will operate at a sub-critical charge. Furthermore, when lubricating oil is trapped in the auxiliary evaporator, the compressor does not receive adequate lubrication resulting in wear and tear of the compressor's internal components. Prior art dual evaporator air conditioning systems attempt to alleviate the buildup of the liquid refrigerant and lubricating oil in the auxiliary evaporator by adding a valve upstream of the auxiliary evaporator. The valve is closed when the auxiliary HVAC unit is in the non-cooling mode and open when the auxiliary HVAC unit is in the cooling mode. Such valves are relatively expensive, and require considerable attention and maintenance to ensure proper operation. As a result, there is a need in the art for an improved, economically feasible system to minimize refrigerant collection in the auxiliary evaporator.